Quiet automatic volume control system



Nov. 26, 1935. W; R, KCH 2,021,939

QUIET AUTOMATIC VOLUME CONTROL SYSTEM Filed May 25, 1954 l 4. E 35 AMR vPatented Nov. 26, 1935 UNITED STATES MTENT oFFlcE QUIET AUTOMATIC VOLUMECONTROL SYSTEM Winfield n. Koch, camini,y N. J., assigner to', v RadioCorporation of America, a corporation of Delaware 5 claims. (ci. 25o-20) My present invention relates to automatic volume control circuits,and more particularly to an improved type of quiet automatic volumecontrol system.

One of the main objects of my present invention is to provide anautomatic volume` control arrangement for a radio receiver, the volumecontrol arrangement having its action supplemented by a quieting, ormuting', device adapted to render the transmission of signal energythrough the receiver inefcient whenever the amplitude of the signalwaves impressed upon the receiver decreases below a predeterminedintensity level, and the muting device deriving its signal energy inputfrom a point in the signal receiver which has a sharper selectivitycharacteristic than the point in the receiver from which the automaticvolume control arrange`Y ment derives its signal energy.

Another important object of the invention is to provide in a radioreceiver anautomatic volume control circuit and a supplementing mutingcircuit, the automatic volume control circuit be'- ing provided with asignal input network having a resonance curve characteristic of the bandpass type, and the muting circuit being provided with a signal inputcircuit which has a relatively' sharper resonance curve characteristic,whereby the action of the muting circuit is eliminated only when asignal of sufcient intensity' is tuned to the mid-peak frequency of themuting circuit characteristic.

Still another object of the 'present invention is to provide in asuperheterodyne receiver an intermediate frequency transmission networkbetween the` second detector and a preceding stage` of intermediatefrequency amplification, the transmission network comprising. aplurality of vresonant circuits tuned to the operating `intermediatefrequency, the receiver additionally'VV including an automatic volumecontrol circuit arranged to be fed by the last resonant circuit of thesaid transmission network, there .being additionally provided a noisesuppressor tubearranged to be fed Ywith signal energy from the input ofsaidl transmission network, and said noise suppressor tube functioningto render the audio amplifier of the receiver operable only when asignal of suicient intensity is tuned in and the A signal frequencycorresponds to the middle peak quency coupling network in Fig. 1.

of the selectivity curve of the first of said resonant circuits,'andthis middle peak corresponding to the mid-frequency of the relativelybroader resonance curve of the transmission network.

The novel features which I believe to be characteri'stic of my inventionare set forth in particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation,will best be understood by reference to the following description, takenin connection with the drawing, in which I have indicateddiagrammatically a circuit organization wherebymy invention may becarried into effect.

In the drawingz^ 15 g. Fig. 1 dagrammatically shows. a superheterodynereceiver embodying the presentinvention, Fig. A2 graphicallyillustratesthe resonance curve characteristics` of the intermediate' fre- Referringnow to the accompanying drawing, there is shown in Fig. l, in purelydiagrammatic manner, that portion of a superheterodyne receiverassociated with my present invention. Only the last stage ofvintermediate frequency z amplification', the second detector and thefirst stage of audio frequency amplification are shown, because allother elements of the receivingfsystem are sufficiently well known tothose skilled in the art. For this reason, the network preceding theintermediate frequency ampliertube Iv has beenfconventionallyrepresented, andV it will be understood that the numeral 2 designatesany well 'known source of intermediate frequency signal energy. Such asource, for example,- may include the usual tuned radiofrequency-amplifier network followed by a local oscillator andfirstdetector tube, or a combined-detector-os-l cillatortube, theoutputof the mixing circuit being followed by one .orr more stages ofintermediate frequency ampliiication.

In any case, the numeral 3 denotes the intermediate frequency couplingtransformer, whose primary and secondary windingsare eachtuned to theoperating intermediater frequency, com- S monly disposed between theinput 'electrodes of the last intermediate frequency amplifiertube l andthe next preceding tube. T

The numeral 4 designates an electron discharge.' i

triode, and those skilled in the art are well acquainted with the factthat the tube comprises, in addition to the usual cathode control gridand plate, a pair of diode anodes 5 and 6. 'Ihe electron stream betweenthe cathode and both diode anodes, and the electron stream between thecathode, grid and plate are independent. The audio amplifier tube 1 hasits control grid connected to the plate of tube 4 through a couplingcondenser 8, a resistor 9 being disposed in the plate voltage supplylead of tube 4.

The operating potentials for the various electrodes of the receivertubes are supplied from a power supply network generally vdenoted by thereference letter S. This supply network includes the customarytransformer Il) whose primary is connected to the alternating currentsupply, and whose secondary is connected between the anodes of the fullwave rectifier tube Il. The usual filter network S has yits inputterminals connected between an intermediate tap on the secondary of thetransformer I3 and the cathode of the rectifier I I; a bleeder resistorI2 being connected across the output terminals of the network S. Thegrounded side of the filter network includes the usual series chokes,and if desired one of these chokes may also function as the fiel-d coilof the loud speaker of the system. A plurality of shunt condensers areconnected across the bleeder resistor I2, and the cathode of tube 4 isconnected to a point intermediate the series chokes of filter S througha resistor I3. A condenser $4 is connected between the cathode side ofresistor I3 and the positive potential side of bleeder resistor I2.

As is well known to those skilled in the art, the various electrodes ofthe tubes used in the receiving system may be connected to appropriatepoints along the bleeder resistor I2, and it will be noted that theplates of tubes l and 4 are connected to the -l-B terminal of thebleeder resistor, while the screen grid of tube I, for example, would beconnected to the point -l-D on the bleeder resistor.

The intermediate frequency signal energy is impressed between the signalgrid an-d cathode of tube I, the amplified output of the tube istransmitted through a coupling network comprising a tuned circuit A, atuned circuit B, and a tuned circuit C coupling the'circuits A and B.Each of these three circuits is individually tuned to the operatingintermediate frequency, and it will be noted that the tuned circuit A isconnected in the plate circuit of tube I, while the `tuned circuit B isconnected in a series path between the diode anode 5 and the cathode oftube 4, the series path including the tuned circuit and a load resistorI5. The tuned circuit C includes a coil I6 magnetically coupled .to thetuned circuit A and a coil I6' magnetically coupled to the circuit B,both these coils being connected in series with a condenser I1. The lowalternating potential sideof circuit C is grounded, and a resistor I8 isconnected across the tuned circuit A. The function of resistor I8 is tocontrol the damping of network A-C--B. The diode dem'odulator also hassome effect on the damping.

The grid of tube 4 is connected to the negative side of resistor I 5through a resistor I9, and a radio frequency by-pass condenser 2!! isconnected in shunt with resistor I5. Any small radio frequency voltagesexisting across condenser 20 are attenuated by this resistor I9, incombina- -ton with the input capacity of the tube 4, so that thesevoltages will not reach the grid of the tube in any appreciable amount.

The diode anode 6 is connected to lead 25, also designated by thereference letters A. V. C. to denote the automatic volume controlconnection; 5 the anode 6 being connected to ground through a resistor2|, and a resistor 22 being included between the lead 25 and the anode6. A direct voltage lter resistor 23 is included in lead 25, and it willbe understood that the lead 25 is to 10 be connected to the gridcircuits of the Various high frequency amplifier stages, one suchconnection of the grid circuit of intermediate frequency amplifier Ibeing designated by the numeral 24. Both resistors 22, 23 function tofilter l5 the audio component from the direct voltage component; onlyone resistor may be used if desired.

The numeral 30 denotes the muter, or noise suppressor, tube which may beof the 2B7 type. 20 rlhis tube includes at least one diode anode 3|connected through a signal energy by-pass condenser 32 to the platecircuit of the intermediate frequency amplifier tube I. It will be notedthat the diode anode 3I is disposed adjacent a portion 25 of the cathodeof tube 30, and that the electron stream from the cathode to this diodeanode is independent of the electron stream flowing from the cathode tothe plate through the interposed three grids. 'I'he grid 33, disposedbetween the 30 Asignal grid and the suppressor grid, is maintained atground potential, an-d is also connected to the plate of tube 30 throughthe resistor 34. The plate side of resistor 34 is connected through apath which includes lead 35 and resistor 35' to 35 the signal grid ofaudio amplifier tube 1. This lead 35 is also designated by the referenceletters A. N. S. to denote that this is the automatic noise suppressorpath, and that this connection controls the effective bias on the gridof audio 40 amplifier tube 1.

The cathode of tube 30 is connected through lead 36 to the negative sideof resistor I3. A resistor 36 is connected between the diode anode 3Iand the cathode of the muter tube, a path 5 including series arrangedresistor 31 and condenser 38 being connected in shunt across resistor36. The grid immediately adjacent the cathode of tube 30 is connected tothe condenser side of resistor 31. The latter resistor filters the 50audio and radio frequency voltage from the bias voltage, so that thepentode portion of tube 30 works only at a. sub-audible rate. The dropacross the resistor furnishes the bias for the pentode portion of tube30. In other words, it serves 55 as a leak for charges on condenser 32.

In order to clearly understand the operation of the present receivingarrangement, it is pointed out that in Fig. 2 there is shown theresonance curve characteristics of ther/coupling network A, 50 C, B atcircuit A andat circuit B. It will be observed that the dotted linecurve A represents the characteristic at circuit" A. In laboratorymeasurements the outside peaks of the selectivity curve A were found tobe separated from the 65 middle peak, which middle peak is located atthe operating frequency of the system, sufficiently in frequency so thatthe selectivity of the rest of the receiver prevents any response tothese' outside peak frequencies. The side peaks of this 70 curve A wouldbe completely eliminated by the selectivity of the preceding I. F.transformer 3, which would have a curve of the same general shape as B,Assuming that it would have the same curve as B, there is shown aselectivity Curve D'between the source '2 and the diode ,3| of tube 30without side peaks. The selectivity at circuit B may be either double ortriple peaked, and in Fig. 2 it is sho-wn double peaked, depending onthe resistance I8 across the primary circuit A.v

Through the action of the automatic volume control, `the .selectivity ofthe receiver, and the selectivity of the coupling system disclosed, thenoise suppressor can be made to render the audio amplifier 1 operableonly when a signal of sufficient intensity is tuned to the middle peakof the selectivity curve A, which, as stated before, is the center ofthe band selected by the receiver. 'The selectivity of the three coupledcircuits is substantially flat topped, as shown in Fig. 2, and thevoltage across the primary circuit A will tend to be quite selective.The automatic volume control arrangement is operated from the output ofthe three coupled circuits, while the noise suppressor, or muter,network is operated from the input of the three coupled circuits. Thismode of operation of the two control circuits insures an accurate tuningof the receiver, since the muting action will not be eliminated untilthe receiver is tuned to the desired signal frequency. Of course, thevariable tuningmeans commonly disposed in th'e network 2 ahead of theintermediate frequency amplifier has not been shown, since those skilledin the' art are well acquainted with such devices.

Considering the precise operation of the automatic volume control andmuter networks, it is first pointed out that the noise suppressor tube30 utilizes the voltage drop across the speaker eld as screen and platevoltages, by virtue of the connection of these two electrodes to theground side of bleeder l2. When the receiver is tuned to the desiredsignal frequency, intermediate frequency energy is impressed upon tunedcircuit A, and such energy is transmitted through condenser 32 and isimpressed between the diode anode 3| and the cathode of tube 3l). Thediode system of the muter tube rectifies the impressed signal Voltageand biases off the pentode portion of the tube 30. This results in adecrease 'in the flow of current through the plate .circuit of tube 30,which reduces the normal cut-off bias applied through lead 35 andresistor 35 to the grid of audio amplifier tube 1. In other words, whensignal voltage is not impressed upon circuit A', the flow of platecurrent through lead 35 is at a maximum and this results in a cut-offbias vbeing applied to the signal grid of amplifier 1.

With respect to the operation of the automatic volume control network ofthe receiver, it is pointed out that this network operates in thefollowing manner: A signal on the diode 5 causes a voltage drop acrossresistor l5. This voltage is impressed on the grid of the triode portionof the tube 4, making the grid more negative. The

resulting decrease in plate current reduces the voltage drop acrossresistor I3, making the cathode of tube 4 go from a positive voltage toa negative voltage with respect to ground. Diode 6 is maintained atground potential by resistor 2| until the cathode becomes negative, andcurrent flows through the diode-cathode path. When this occurs, diode Gand the grids of the controlled tubes follow very closely the cathodevoltage of tube 4. The stronger the signal, the more negative thiscathode becomes, and because the grids of the amplifiers become morenegative the less will be the gain in the controlled tubes.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organizations shownand described but that many 5e modifications may be made withoutdeparting from the scope of my invention, as set worth in the appendedclaims.

What I claim is: v

1. In combination with a high frequency ampli- 10 fier and an automaticvolume control network for regulating the gain of the amplifier inresponse to variations in the amplitude of signals impressed on theamplifier, a coupling network connected between the amplifier and theautomatic volume l5 control arrangement, a signal transmission tubehaving its input electrodes coupled to the. output of said couplingnetwork, a muter tube having an output electrode connected to the inputcircuit of said signal transmission tube, the muter tube having amaximum flow of space current when signals below a predeterminedamplitude are impressed upon said high frequency amplifler, a signalpath connecting an input electrode of said muter tube to the input ofsaid coupling network, and said coupling network including a pluralityof resonant circuits tuned to an operating signal frequency, saidcoupling network being constructed in such a manner that the resonancecurve characteristic at the output of the coupling network issubstantially broader than the resonance curve characteristic at theinput of said network.

2. In a system as defined in rclaim 1, said coupling network consistingof three coupled resonant circuits tuned to the said operating signalfrequency, the resonance curve characteristic of the coupling network atthe last resonant circuit being of the band pass type, and the resonancecurve characteristic at the first of said resonant circuits beingrelatively more selective and having a sharp peak located at themid-frequency of said band pass characteristic.

3. In a system as defined in claim l, said muter tube including at leastone diode anode disposed adjacent its cathode, the diode anode andcathode including a load impedance whereby a diode rectier circuit isprovided, and said signal path connecting said diode anode to the inputof said couplingnetwork.

4. In a superheterodyne receiver of a type including an intermediatefrequency amplifier, a second detector, an audio frequency amplifier,and anA automatic volume control circuit electrically associated withthe second detector and the grid circuit of said intermediate frequencyamplifier, a coupling network connected between the plate circuit ofsaid intermediate frequency amplifier and the second detector, a mutertube having an input electrode coupled to the input of said couplingnetwork and an output electrode directly connected to the grid circuitof said audio amplifier, said coupling network including a suificientnumber of circuits tuned to the operating intermediate frequency toimpart a relatively broad resonance curve characteristic to the circuitfeeding said second detector, and the circuit feeding said muter tubehaving a resonance curve characteristic characterized by a relativelysharp peak located at the mid-band 'frequency of said first 70characteristic.

5. In a radio receiver of the superheterodyne, type, an intermediatefrequency transmission network including a plurality of tuned circuitsarranged in cascade, a demodulator coupled to the last of the cascadedcircuits, an audio frequency transmission network arranged to receivedemodulated signal energy from the demodulator, means responsive tosignal amplitude variations at the demodulator input for automaticallyregulating the eciency of signal transmission through the intermediatefrequency network, means, responsive to signal amplitude Variations at apoint in the intermediate frequency `network preceding said last of thecascaded circuits, for automatically regulating the eiciency of signaltransmission through the audio network, the resonance curvecharacteristic of the intermediate network at said lastoircuit beingsubstantially broader than the characteristic at said preceding point inthe net- Work.

WINFIELD R. KOCH.

